A fiber Bragg grating is a distributed Bragg reflector constructed in a waveguide such as in the core of an optical fiber (FIG. 1). The Bragg grating is created by periodically varying the refractive index of the core of the optical fiber. The Bragg grating thus acts as a wavelength-specific dielectric mirror, blocking or reflecting certain wavelengths of radiation passing along the waveguide. The blocked or reflected Bragg wavelength typically varies in a known manner with ambient temperature, rendering FBG's applicable as temperature sensors.
Bragg fiber gratings have been made in several ways, whereby the periodic variation is created by irradiating a photosensitive optical fiber. The irradiation source is usually a high intensity UV laser. The specific pattern of the Bragg grating is typically created by interference, use of a photomask or point by point inscription by a narrow focus laser beam.
Although FBG are usually referred to as a permanent refractive index modulation in the fiber core, exposure to high-temperature environments may result in the bleach and eventual erasure of the refractive index modulation, especially at extended time periods.
The maximum usable temperature for conventional FBGs is around 600° C. due to the weak bonds of germanium and oxygen in the optical fiber. Various methods have been employed in an attempt to increase the maximum usable temperature, including accelerated aging, pre-irradiation, formation of type II (damage inscribed) gratings, specialist ion-doped fibers and chemical composition fibers. Recently, FBGs fabricated by use of femtosecond laser pulses have exhibited enhanced thermal stability, up to about 1000° C. Such FBGs are referred to as type II damage written gratings, since the ultra high peak power locally affects the glass structure of the fiber. Type II-IR gratings are created when the laser intensity is greater than the damage threshold of the particular glass.
However, the thermal stability still can not be maintained when the temperature is increased to 1100° C. or higher. This has severely limited the applicability of FBG's as high temperature sensors.